[clue] Extending files with fallocate.

[David L. Anselmi]

So the way to make big files quickly is like this:

fallocate -l 2G foo

(Makes a 2GB file called foo.  Allocates the data blocks but does not write to them.  Requires a 
file system like ext4.)

If you're then putting data into foo and run out of space (say that foo is loop mounted and contains 
a partition table and file systems of its own) you can extend it like this:

fallocate -l 4G foo

That will not touch any of the existing data.  (That isn't documented well that I could find so I'm 
letting you know.) (It's probably a good idea that foo isn't open when you extend it.)

Dave